My 2000 s10 pickup has low to no heat. Had garage flush out. Was told needs a new radiator because the old one was filled with junk. Put in new radiator and thermostat $400 - still little to no heat. Garage flushed out again, ran cleaning fluid through and checked the water pump. They then told me the heater core was plugged and I needed a new one. After they put in a new heater core for 800$ I had full heat for a few days then back to little or no heat. Called garage and was told fluid might be low. Checked fluid and it was not low (I was able to add about 1 cup of antifreeze). The line going into the heater core is hot the one coming out is cold so its not circulating. It seems like my new heater core is plugged or there is a air lock. I'm not sure if I should bring it back to the same garage so they can not fix it while maxing out my credit card. Was thinking of taking it to a Chevy dealer to see if they can find the problem. Any suggestions would be greatly appreciated.

Most common cause of no heat from a heater core is a clogged core, not the radiator. Take off the exit side of the heater core hose, put a garden hose in. Disconnect the engine supply side and flush backwards to the normal flow. Ford Rangers are notorious for this issue and a backflush works wonders. Good luck.

You've done a lot of good things to your truck. I suspect the heater control valve is bad, preventing coolant from circulating through the heater core. I believe the heater temperature blend valve is located behind the dash assembly. The dash assembly must be removed to gain access. Take it to Chevrolet for work.

A tip I always try to pass along is to only refill your cooling system with a 50/50 solution of the recommended antifreeze and distilled water. Distilled water prevents the build-up of phosphate scale (minerals) forming in the radiator, heater core, and heater control valve. Change the coolant every 2 to 3 years.